Also to anyone following this thread - in regards to Teensy4:

I was offered a T4 beta board back when it started, but I could not accept as I did not have the requisite time to work on a library port.

I have now obtained a couple of the retail boards, and although I still lack time, I will probably attempt a port as I can get to it. I have not reviewed Wire code on T4 to know how different it is, but I did read something about it using a FIFO, so my guess is the ISR code would be different enough to warrant a new lib (eg. i2c_t4). I am uncertain of interest level, if any, so any comments would be helpful.